Various methods are proposed in order to calculate a residual capacity of a battery.
Patent Document 1 (Japanese Unexamined Patent Publication No. 2009-51351) discloses a vehicle power-supply device as described below. First, an integrated value S obtained by integrating a square value of a minimum voltage value from an open voltage value equivalent to a residual capacity 0 to an open voltage value of full charge is calculated on the basis of an open voltage value of an in-vehicle battery and a minimum voltage value of the in-vehicle battery at the time of engine start-up, and a table is created. In addition, a coefficient K of the integrated value S and electric energy E of full charge of the in-vehicle battery is calculated on the basis of an actual measured value and is stored. During the use of the in-vehicle battery, first, referring to the table from the actual measured value of the minimum voltage value, the integrated value S is obtained. Next, the integrated value S is multiplied by the coefficient K, and the residual capacity of the in-vehicle battery is calculated to electric energy (unit: Wh).